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[PATCH v3 11/12] s/recommendable/recommended 486+ messages / 2 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 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 7/8] Decouple updating of freezing information from swap_relation_files(). @ 2026-07-15 08:37 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-07-15 08:37 UTC (permalink / raw) So far, the relfrozenxid and relminmxid attributes of pg_class used to be updated by swap_relation_files(), however, not every call of the function should do that. On the contrary, new feature of REPACK (CONCURRENTLY) will need to update this information in one case that would be tricky to handle using swap_relation_files(). --- src/backend/commands/repack.c | 114 +++++++++++++++++----------------- 1 file changed, 56 insertions(+), 58 deletions(-) diff --git a/src/backend/commands/repack.c b/src/backend/commands/repack.c index 89695335127..7390fd303f6 100644 --- a/src/backend/commands/repack.c +++ b/src/backend/commands/repack.c @@ -188,6 +188,8 @@ static void copy_table_data(Relation NewHeap, Relation OldHeap, Relation OldInde TransactionId *pFreezeXid, MultiXactId *pCutoffMulti, ChangeContext *chgcxt); +static void update_relation_cutoffs(Oid relid, TransactionId frozenXid, + MultiXactId cutoffMulti); static List *get_tables_to_repack(RepackCommand cmd, bool usingindex, MemoryContext permcxt); static List *get_tables_to_repack_partitioned(RepackCommand cmd, @@ -2039,13 +2041,6 @@ copy_table_data(Relation NewHeap, Relation OldHeap, Relation OldIndex, * links) while the latter is the only way to handle cases in which a toast * table is added or removed altogether. * - * Additionally, the first relation is marked with relfrozenxid set to - * frozenXid. It seems a bit ugly to have this here, but the caller would - * have to do it anyway, so having it here saves a heap_update. Note: in - * the swap-toast-links case, we assume we don't need to change the toast - * table's relfrozenxid: the new version of the toast table should already - * have relfrozenxid set to RecentXmin, which is good enough. - * * Lastly, if r2 and its toast table and toast index (if any) are mapped, * their OIDs are emitted into mapped_tables[]. This is hacky but beats * having to look the information up again later in finish_heap_swap. @@ -2054,9 +2049,7 @@ static void swap_relation_files(Oid r1, Oid r2, bool target_is_pg_class, bool swap_toast_by_content, bool is_internal, - TransactionId frozenXid, - MultiXactId cutoffMulti, - Oid *mapped_tables) + Oid *mapped_tables, Oid *r1_toastid) { Relation relRelation; HeapTuple reltup1, @@ -2088,6 +2081,9 @@ swap_relation_files(Oid r1, Oid r2, bool target_is_pg_class, relam1 = relform1->relam; relam2 = relform2->relam; + if (r1_toastid) + *r1_toastid = relform1->reltoastrelid; + if (RelFileNumberIsValid(relfilenumber1) && RelFileNumberIsValid(relfilenumber2)) { @@ -2203,15 +2199,6 @@ swap_relation_files(Oid r1, Oid r2, bool target_is_pg_class, * and then fail to commit the pg_class update. */ - /* set rel1's frozen Xid and minimum MultiXid */ - if (relform1->relkind != RELKIND_INDEX) - { - Assert(!TransactionIdIsValid(frozenXid) || - TransactionIdIsNormal(frozenXid)); - relform1->relfrozenxid = frozenXid; - relform1->relminmxid = cutoffMulti; - } - /* swap size statistics too, since new rel has freshly-updated stats */ { int32 swap_pages; @@ -2313,9 +2300,8 @@ swap_relation_files(Oid r1, Oid r2, bool target_is_pg_class, target_is_pg_class, swap_toast_by_content, is_internal, - frozenXid, - cutoffMulti, - mapped_tables); + mapped_tables, + NULL); } else { @@ -2416,9 +2402,8 @@ swap_relation_files(Oid r1, Oid r2, bool target_is_pg_class, target_is_pg_class, swap_toast_by_content, is_internal, - InvalidTransactionId, - InvalidMultiXactId, - mapped_tables); + mapped_tables, + NULL); } /* Clean up. */ @@ -2428,6 +2413,36 @@ swap_relation_files(Oid r1, Oid r2, bool target_is_pg_class, table_close(relRelation, RowExclusiveLock); } +/* + * Update relfrozenxid and relminmxid attributes of pg_class entry, specified + * by OID. + */ +static void +update_relation_cutoffs(Oid relid, TransactionId frozenXid, + MultiXactId cutoffMulti) +{ + Relation relRelation; + HeapTuple reltup; + Form_pg_class relform; + CatalogIndexState indstate; + + relRelation = table_open(RelationRelationId, RowExclusiveLock); + reltup = SearchSysCacheCopy1(RELOID, ObjectIdGetDatum(relid)); + if (!HeapTupleIsValid(reltup)) + elog(ERROR, "cache lookup failed for relation %u", relid); + relform = (Form_pg_class) GETSTRUCT(reltup); + relform->relfrozenxid = frozenXid; + relform->relminmxid = cutoffMulti; + + indstate = CatalogOpenIndexes(relRelation); + CatalogTupleUpdateWithInfo(relRelation, &reltup->t_self, reltup, + indstate); + CatalogCloseIndexes(indstate); + + heap_freetuple(reltup); + table_close(relRelation, RowExclusiveLock); +} + /* * Remove the transient table that was built by make_new_heap, and finish * cleaning up (including rebuilding all indexes on the old heap). @@ -2445,6 +2460,7 @@ finish_heap_swap(Oid OIDOldHeap, Oid OIDNewHeap, { ObjectAddress object; Oid mapped_tables[4]; + Oid oid_old_toastid; int i; /* Report that we are now swapping relation files */ @@ -2461,7 +2477,7 @@ finish_heap_swap(Oid OIDOldHeap, Oid OIDNewHeap, swap_relation_files(OIDOldHeap, OIDNewHeap, (OIDOldHeap == RelationRelationId), swap_toast_by_content, is_internal, - frozenXid, cutoffMulti, mapped_tables); + mapped_tables, &oid_old_toastid); /* * If it's a system catalog, queue a sinval message to flush all catcaches @@ -2517,37 +2533,20 @@ finish_heap_swap(Oid OIDOldHeap, Oid OIDNewHeap, PROGRESS_REPACK_PHASE_FINAL_CLEANUP); /* - * If the relation being rebuilt is pg_class, swap_relation_files() - * couldn't update pg_class's own pg_class entry (check comments in - * swap_relation_files()), thus relfrozenxid was not updated. That's - * annoying because a potential reason for doing a VACUUM FULL is a - * imminent or actual anti-wraparound shutdown. So, now that we can - * access the new relation using its indices, update relfrozenxid. - * pg_class doesn't have a toast relation, so we don't need to update the - * corresponding toast relation. Not that there's little point moving all - * relfrozenxid updates here since swap_relation_files() needs to write to - * pg_class for non-mapped relations anyway. + * Update relfrozenxid and relminmxid. CCI is needed to see the most + * recent pg_class tuple, created by swap_relation_files(). However that + * CCI makes us use the new relation file. Therefore we cannot do this + * before indexes have been rebuilt too. */ - if (OIDOldHeap == RelationRelationId) - { - Relation relRelation; - HeapTuple reltup; - Form_pg_class relform; - - relRelation = table_open(RelationRelationId, RowExclusiveLock); - - reltup = SearchSysCacheCopy1(RELOID, ObjectIdGetDatum(OIDOldHeap)); - if (!HeapTupleIsValid(reltup)) - elog(ERROR, "cache lookup failed for relation %u", OIDOldHeap); - relform = (Form_pg_class) GETSTRUCT(reltup); - - relform->relfrozenxid = frozenXid; - relform->relminmxid = cutoffMulti; - - CatalogTupleUpdate(relRelation, &reltup->t_self, reltup); + CommandCounterIncrement(); + update_relation_cutoffs(OIDOldHeap, frozenXid, cutoffMulti); - table_close(relRelation, RowExclusiveLock); - } + /* + * The same for TOAST, if needed. In the swap-toast-links case, the new + * the toast table should already have relfrozenxid set to RecentXmin. + */ + if (OidIsValid(oid_old_toastid) && swap_toast_by_content) + update_relation_cutoffs(oid_old_toastid, frozenXid, cutoffMulti); /* Destroy new heap with old filenumber */ object.classId = RelationRelationId; @@ -4133,9 +4132,8 @@ rebuild_relation_finish_concurrent(Relation NewHeap, Relation OldHeap, (old_table_oid == RelationRelationId), false, /* swap_toast_by_content */ true, - InvalidTransactionId, - InvalidMultiXactId, - mapped_tables); + mapped_tables, + NULL); #ifdef USE_ASSERT_CHECKING -- 2.52.0 --=-=-= Content-Type: text/plain Content-Disposition: attachment; filename=v02-0008-Make-REPACK-CONCURRENTLY-MVCC-safe.patch ^ permalink raw reply [nested|flat] 486+ messages in thread
end of thread, other threads:[~2026-07-15 08:37 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]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-07-15 08:37 [PATCH 7/8] Decouple updating of freezing information from swap_relation_files(). Antonin Houska <[email protected]>
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